New test IDs TB infections in less than 2 hours

A new, automated nucleic acid–amplification test might take the time to diagnose TB infection, even a drug resistant strain, down to 90 minutes. That’s according to a study published in The New England Journal of Medicine Wednesday.

A Zambian clinical worker tests for TB using a sputum smear microscopy, which routinely misses about 50 percent of all TB cases.

The Xpert MTB/RIF tests for Mycobacterium tuberculosis (MTB) and resistance to rifampin (RIF), a powerful anti-TB drug. Trial researchers tested 1730 patients suspected of TB infection over a nine month period at four trial sites in Lima, Peru; Baku, Azerbaijan; Cape Town and Durbin, South Africa; and Mumbai, India. The test successfully identified 98 percent of all culture-confirmed TB cases, including more than 90 percent of those whose sputum smears tested negative for acid-fast bacilli. The Xpert also accurately detected more than 97 percent of patients with rifampin-resistant bacteria in less than two hours.

The results were slightly less impressive for those who were HIV/TB coinfected, showing a 93.9 percent TB detection rate, compared with 98.4 percent in patients that were HIV-negative. In contrast, the most common means of testing for TB at present – the sputum smear microscopy – is not reliable among HIV positive patients.

TB infection is the biggest killer of people living with HIV/AIDS, and kills approximately 1.8million people worldwide every year. Drug-resistant strains of the bacterium presented in about 440,000 people and killed 150,000 last year alone.

The simple Xpert test could be performed by relatively unskilled health care workers, according to researchers, reducing the time it takes to get test results by days or even weeks. Quicker test results means quicker treatment for those who test positive, potentially reducing the spread of TB to others significantly.

While the test is speedy and accurate, the Xpert utilizes “sophisticated technology, which is costly to manufacture,” as stated in the study text. So while it would be relatively easy to use, low-income countries would need to fork out considerably more money for Xpert testing than the roughly $8 it costs for multiple conventional tests including smear plus culture or multiple rounds of smear, according to Nature News.

“However, the Xpert could be less costly than implementation of culture and drug-susceptibility testing,” the study states.

It is also important to note that “only reference facilities were used in the study, and it is not certain that our findings would be replicated in microscopy centers, health posts, and other point-of-treatment settings where temperature and electricity supply will be more variable and training issues will be more relevant,” according to the study authors. Annual calibration of the machine was also an issue, especially at rural testing sites.

“Large-scale projects to show the feasibility and effect of MTB/RIF testing at such sites are under way,” they said.

“Over the next few weeks, [the World Health Organization (WHO)] will be convening independent experts to review the evidence about the efficacy and field effectiveness of this novel diagnostic technology,” said Lakshmi Sundaram, from the Foundation for Innovative New Diagnostics (FIND) in an email. “We hope to have a recommendation regarding its widespread use following a meeting in end-September of WHO’s Scientific and Technical Advisory Group.”

FIND is a co-developer of the test along with partners Cepheid and the University of Medicine and Dentistry of New Jersey. The National Institute of Allergy and Infectious Diseases and the Bill & Melinda Gates Foundation also participated in the study.

The WHO said in a statement that the results from the Xpert test trial suggest that it has “the potential to revolutionize TB care, and WHO will treat it as a top priority.”